The technology involves matching a person’s Web identity with information gathered about that person offline, including his or her party registration, voting history, charitable donations, address, age, and even hobbies.

Companies selling political targeting services say “microtargeting” of campaign advertising will make it less expensive, more up to date, and possibly less intrusive. But critics say there are downsides to political ads that combine offline and online data, and not just a possible invasion of privacy. “These are not your mom-and-pop TV ads. These are ads increasingly designed for you—to tell you what you may want to hear,” says Jeff Chester, executive director of the Center for Digital Democracy.

Audience Partners, a firm founded in 2008, says it can now reach 130 million registered U.S. voters online, or about 80 percent of total voters. The company was formed after one of its founders ran for Congress in the Philadelphia area and calculated that he would have to spend $3 million on television ads running in a TV market of six million people in order to reach a district with just 367,000 registered voters.

The firm gathers publicly available voter files from all 50 states and supplements this information with records of political donations and other profiles purchased from commercial data brokers, says CEO Jeff Dittus. Then, working with about 100 high-traffic websites that register their users, they can match the offline data to the online identities of individuals.

Few Web surfers realize how widely data about them gets bought, sold, and combined. But the practice is common. In a recent investigation, ProPublica revealed that Microsoft and Yahoo each offer political campaigns the ability to target voters in similar ways.

Cookies are short bits of code that identify a person’s browser. With the help of advertising exchanges and media partners, a political campaign can use cookies to serve specific ads to, for example, all registered 50-to-60-year-old male Democrats in Pennsylvania’s 6th district who are frequent voters and care about the environment. A campaign could even see whether specific individuals click on the ad and what they do once on its landing page.

The techniques may also let politicians quickly test and alter political messages to learn which resonate most. That may let them deliver ads that are ever more “relevant”—a term used frequently in e-commerce but only more recently in politics. Different ads, even negative ones, could be shown to people identified as Republicans, Democrats, or swing voters.

Dittus says online advertising is also becoming a form of real-time polling. For instance, during New Jersey governor Chris Christie’s 2009 campaign, Audience Partners supplied data to Campaign Grid, a Republican online tracking firm. In the week before the election, Dittus says, the rate at which people clicked on different ads corresponded closely to the top four voter issues as measured in exit polls: jobs, property taxes, corruption, and health care.

Most major campaigns this year will use cookie-based political advertising, says John Phillips, CEO of Aristotle International, which sells voter records combined with information like people’s hobbies and income brackets. Last week, Phillips announced a partnership with the online advertising firm Intermarkets that will let political campaigns select their online audiences based on more than 500 demographic and behavioral factors.

While political speech has always been tailored to the audience—the Obama and Romney campaigns, for example, could run different television commercials in Ohio and Florida—the level of personalization online is far greater and less transparent to the voter.

Chester, the watchdog at the Center for Digital Democracy, says online political targeting is “antithetical to the democratic process.” He believes it could lead to an increase in misleading ads, even ones that “may not reflect the actual positions of the candidate.” With so many different ads in circulation, it could be hard for the news media to check their accuracy.

Chester thinks added caution is warranted in the political context, just as it is for sensitive medical and financial data. Campaigns should ask for people’s permission before using personal profiling and tracking to target them with political messages, and Congress or the Federal Election Commission should regulate the practice, he says.

Aristotle’s Phillips also says there will be longer-term implications. Other countries have already moved to online voting in some cases, and the U.S. may one day as well. As this happens, he says, the ability to communicate with voters, possibly right up to their moment of decision, will become more important.

“The debate about new technology and targeting and the like is really about political speech, and the way that decisions at the ballot box are influenced,” he says. “How you influence a person’s decision is really being tested this election.”